Re: Prefetch object (django 1.7)

Indeed, I am PostAuthor is a through model. Thanks I understand now how it works.  Prefetch('postauthors', PostAuthors.objects....) fails with
AttributeError: Cannot find 'postauthors' on Post object, 'postauthors' is an invalid parameter to prefetch_related()
So it seems that 'postauthors' is not a valid prefetch lookup.  I don't need it, I just wanted to order Post.authors using the column PostAuthor.created.  Prefechting PostAuthors might need just using PostAuthors.objects directly, or sth like this:

p = Prefetch('authors', User.objects.extra(select={'created': 'created'}).order_by('postauthor__created'))
Post.objects.prefetch_related(p).all()

I think it would be nice to add such examples to the docs.   If that's the case I can write a paragraph with an example like the one I am using, i.e. with a custom through model.

Best regards,
Marcin Szamotulski

On 30 January 2014 05:23, Anssi Kääriäinen <akaariai@gmail.com> wrote:

To me it seems you are mixing the authors m2m relation and the through model. The 'authors' relates to User, not PostAuthor. But your queryset is pointing to PostAuthors.

If you want to fetch PostAuthor instances, then you should likely prefetch with Prefetch('postauthors', PostAuthors.objects...). I am not exactly sure if the 'postauthor'
lookup is right. It should be whatever post = models.ForeignKey(Post)'s reverse name is.

 - Anssi

On Thursday, January 30, 2014 4:45:30 AM UTC+2, Marcin Szamotulski wrote:

On 01:42 Mon 27 Jan     , Marcin Szamotulski wrote:
> Hello,
>
> I have a model
>
> class Post(models.Model):
>
>     ...
>     authors = models.ManyToManyField('accounts.User', through='PostAuthor', related_name='authors_posts')
>
>
> class PostAuthor(models.Model):
>
>     user = models.ForeignKey('accounts.User')
>     post = models.ForeignKey(Post)
>     ...
>
>
> How can I use the Django 1.7 Prefetch object to load PostAuthors,  this
> does not work:
>
> Post.objects.prefetch_related(
>     Prefetch(
>         'authors',
>         queryset=PostAuthors.object.select_related('user')
>     )
>
> I got an exception:
> django.core.exceptions.FieldError: Cannot resolve keyword 'authors_posts' into field. Choices are: created, id, post, post_id,
> user, user_id
>
> Thanks for help,
> Marcin Szamotulski


If somebody will search here is the solution I've found:

Post.objects.prefetch_related(
    Prefetch(
        'authors',
        queryset=User.objects.order_by('postauthor__created'),
    )

It works because when prefetching User table is joined (inner joined)
with PostAuthor table and then the order_by('postauthor__created') will
make sense since PostAuthor has a columnt 'created'.

It would be nice to have an example in the docs for that though.

Best regards,
Marcin

--
You received this message because you are subscribed to the Google Groups "Django users" group.
To unsubscribe from this group and stop receiving emails from it, send an email to django-users+unsubscribe@googlegroups.com.
To post to this group, send email to django-users@googlegroups.com.
Visit this group at http://groups.google.com/group/django-users.
To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/2af033f8-e6b3-44e3-bf35-2f62e9d5fd93%40googlegroups.com.
For more options, visit https://groups.google.com/groups/opt_out.

--
You received this message because you are subscribed to the Google Groups "Django users" group.
To unsubscribe from this group and stop receiving emails from it, send an email to django-users+unsubscribe@googlegroups.com.
To post to this group, send email to django-users@googlegroups.com.
Visit this group at http://groups.google.com/group/django-users.
To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/CADkSB_2gSUi4sUHpwdHGgKGMHZ-xq2QR8gusTM9rOffamoQ0%2BQ%40mail.gmail.com.
For more options, visit https://groups.google.com/groups/opt_out.